Which property states that the order of addition does not affect the result?

A number that has exactly two distinct positive divisors is called:

Which of the following is the locus of points in the plane whose distances from 

(0,0) and  (4,0) differ by 2?

A rectangular pasture must be fenced on all four sides and divided into two congruent rectangles by a fence parallel to one of its shorter sides. If the total length of fencing is 120 m, what dimensions maximize the total area?

Which major work by Gauss laid the foundations of modern number theory, including congruences and modular arithmetic, and influenced much of later algebra?